Recently micro-machined ultrasound transducers (MUT) have been developed. Micro-machined ultrasound transducers have been fabricated in two design a approaches, one using a semiconductor layer with piezoelectric properties (pMUT) and another using a membrane (or diaphragm) and substrate with electrodes (or electrode plates) forming a capacitor, so-called capacitive micro-machined ultrasound transducer (cMUT).
A cMUT cell comprises a cavity underneath the membrane. For receiving ultrasound waves, ultrasound waves cause the membrane to move or vibrate and the variation in the capacitance between the electrodes can be detected. Thereby the ultrasound waves are transformed into a corresponding electrical signal. Conversely, an electrical signal applied to the electrodes causes the membrane to move or vibrate and thereby transmitting ultrasound waves.
Initially, cMUT cells were produced to operate in what is known as an “uncollapsed” mode. The conventional “uncollapsed” cMUT cell is essentially a non-linear device, where the efficiency strongly depends on the bias voltage applied between the electrodes.
In order to solve this problem, so-called “pre-collapsed” cMUT cells have recently been developed. In a pre-collapsed cMUT cell a part of the membrane is permanently collapsed or fixed to the bottom of the cavity (or substrate). Above a certain bias voltage the efficiency of a pre-collapsed cMUT cell is substantially bias voltage-independent, which makes the cMUT cell much more linear.
In the pre-collapsed cMUT cell, the membrane can be collapsed using different methods, for example using electrical or mechanical collapsing.
Electrical collapsing can for example be achieved using the bias voltage. WO 2009/037655 A2 discloses a method for producing a cMUT, comprising providing a nearly completed cMUT, wherein the nearly completed cMUT defines one or more cMUT elements that include: (i) a substrate layer, (ii) an electrode plate, (iii) a membrane layer, and (iv) an electrode ring, defining at least one hole through the membrane layer for each cMUT element, applying a bias voltage across membrane and substrate layers of the one or more cMUT elements so as to collapse the membrane layer relative to the substrate layer, and fixing and sealing the collapsed membrane layer relative to the substrate layer by applying an encasing layer.
Mechanical collapsing can for example be achieved using the ambient air pressure. WO 2010/097729 A1 discloses a cMUT cell comprising a substrate, a first electrode attached to the substrate, a movable membrane formed in spaced relationship to the first electrode, a second electrode attached to the membrane, and a retention member, overlaying the movable membrane when the membrane is in a pre-collapsed state which acts to retain the membrane in its pre-collapsed state in the absence of the bias voltage. In one example, the retention member is cast over the cMUT transducer cell while the membrane is brought to a pre-collapsed state by application of (atmospheric) pressure to the membrane.
Pre-collapsed cMUT cells as disclosed in WO 2010/097729 A1 have been successfully manufactured as low frequency cMUT cells having a relative large diameter membrane. The collapse pressure was low and the cMUT cells were pre-collapsed by ambient air pressure (i.e. the membrane touches the bottom of the cavity). However, for high frequency cMUT cells a retention member as disclosed in WO 2010/097729 A1 cannot be applied, as the collapse pressure is very large and can easily exceed for example 5 Bar or even 10 Bar. In this case, the retention layer as disclosed in WO 2010/097729 A1 is not strong enough to keep the membrane in place. Thus, the problem with the cMUT cells as disclosed in WO 2010/097729 A1 is that it is essentially a “large membrane” solution, but does not work for high frequency cMUT cells, having a small membrane diameter.
There is a need to improve such pre-collapsed capacitive micro-machined transducer cell, in particular for high frequencies.